Quorum sensing (QS) is a type of bacterial cell-to-cell signaling pathway mediated through the production, release and detection of the small signaling molecules called autoinducers (AIs) (reviewed in (1)). Such communication allows bacterial control of crucial functions in united communities for enhancement of symbiosis, virulence, antibiotic production, biofilm formation, and many other processes. The recent increase in prevalence of bacterial strains resistant to antibiotics emphasizes the need for the development of a new generation of antibacterial agents. As QS is utilized by number of pathogenic bacteria to direct virulence and biofilm formation, inhibitors/modulators of QS may serve as tools to study or intercept such community behaviors and might be beneficial as antibacterial agents (2). One of the chemical signals, or autoinducers (AIs) used by Gram-negative bacteria are acyl homoserine lactones (AHLs), which are detected by their cognate regulator (R) proteins (1).
The QS system Vibrio fischeri (recently reclassified as Aliivibrio fisheri) serves as the paradigm upon which all other QS systems are based. It is composed of a transcriptional regulator protein (R) and a synthetase (I). The R protein is unstable unless it binds to the AHL which is produced by the synthetase. V. fischeri produces N-3-(oxo-hexanoyl)-homoserine lactone (3OC6HSL), V. harveyi synthesizes N-3-hydroxybutanoil-homoserine lactone (HAI-1), and Pseudomonas aeruginosa produces two distinct AHLs: N-3-oxo-dodecanoyl-L-homoserine lactone (3-OC12-HSL) and N-butyryl-L-homoserine lactone (C4-HSL). V. harveryi produces a second signalling molecule, a furnosyl borate diester, termed autoinducer-2 (AI-2), and a third CAI-1. CAI-1 has been identified in V. cholerae to be (s)-3 hydroxytridecan-4-one. P. aeruginosa also has a third QS signaling molecule, 2-heptyl-3-hydroxy-4-quinolone (PQS), which is induced and repressed by the las and rhl systems, respectively.
Pseudomonas aeruginosa is an important human opportunistic pathogen affecting immunocompromised individuals, cancer patients, burn victims, cystic fibrosis patients and patients with impaired lung function. It uses two AHL systems called, las and rhl to mediate QS. LasI/R synthesizes and detects N-(3-oxo-dodecanoyl)-L-homoserine lactone (3-oxo-C12-AHL) while RhlI/R synthesizes and detects N-butanoyl-L-homoserine lactone (C4-AHL) (FIG. 1a). In addition, P. aeruginosa has a third QS-dependent pathway, Pseudomonas quinolone signal (PQS) that uses 2-heptyl-3-hydroxy-4-quinolone as an autoinducer (reviewed in (3)). Although, certain genes appear to be regulated by one pathway, for example regulation of genes involved in rhamnolipid synthesis by the rhl pathway (4), there is much overlap between the pathways and what was once thought to be a hierarchical pathway, with las activating rhl, is now known to be much more complex (5). Accumulated evidences clearly indicate the importance of P. aeruginosa QS in disease (6).
Over two decades, several small molecules have been identified by many research groups as inhibitors of the AHL:R protein complex. These are mostly AHL-based structures with moderate changes on the acyl side chain and amide linkage. Some of the most potent inhibitors prepared by Geske and Blackwell are shown in FIG. 1b (7). Recently, Meijler and co-workers designed a ligand, 3, which covalently modified LasR (8). Since AHL is the pharmacophore present in the natural substrates, AHL-based inhibitors are likely to modulate R protein activation.
Studies of structural features other than the AHL scaffold as tools to understand the R type protein interaction with AHLs are limited, although they might aid in rational design of QS inhibitors. Only a few examples of inhibitors with the altered lactone ring structure of AHL have been reported (9-12). For example, Smith et al reported 3-oxo-C12-(2-aminocyclohexanone) (FIG. 1c, 4) as a strong antagonist of LasR system (9), while Muh et al identified two LasR inhibitors having a phenyl and stetrazole ring (e.g. FIG. 1c, 5), with IC50 in nM range (FIG. 1c) (10). It is noteworthy that γ-thiolactone analogue of 1 (FIG. 1b) showed inhibition of LuxR while the corresponding ε-lactam (caprolactam) analogue was reported to lack LuxR binding (13). Thus, a need exists for new QS inhibitors.